This invention relates to pipettes and titrators and, more particularly, to pipettes and titrators having an electrically operated actuator. Specifically, the invention is directed to a self-contained automated air displacement pipette and titrator for portable operation having an electronically controlled digital linear actuator, which accomodates removably attachable displacement assemblies of various sizes and compensates for errors inherent in operation, thereby providing improved precision and accuracy.
Electrically operated linear actuators for controlling displacement piston movement in a pipette are known. However, in order to effectively use a pipette having an electrically operated linear actuator in a laboratory, a portable instrument approaching the size, shape, and weight of known mechanically operated pipettes is desirable.
In this regard, the size and shape of the pipette is critical to portability. If the pipette is overly long or has a large diameter, the instrument is unwieldy. Heretofore, electrically operated pipettes have been configured so that a stepper motor is typically attached directly to and adds directly to the length of the linear actuator shaft, as disclosed in Nishi, U.S. Pat. No. 3,915,651, and Klein, U.S. Pat. No. 4,399,711. The automatic pipettes disclosed in these patents are configured so that a stepper motor is in piggyback relation to the actuator shaft with the drive shaft of the stepper motor connected to the end of the actuator shaft, which substantially increases the length of these pipettes. Such construction cannot be considered suitable to a portable hand-holdable application in which high dexterity is needed to perform rapid motions between wells on a tray used in medical diagnostic tests or between more distant test stations. The construction of these pipettes, furthermore, is not suitable to reach into test tubes.
Also, the automatic pipette disclosed in Citrin, U.S. Pat. No. 4,369,665, includes a threaded screw driven by a motor incorporated into the pipette in side-by-side relation to the piston, which increases the width or diameter of the pipette. Consequently, the pipette disclosed in this patent is bulky.
A further consideration of portability for pipettes is weight. However, considerable energy is required by known pipettes having a linear actuator driven by a stepper motor. For example, in order to hold stepper motors in position, continuous power is typically needed. Heretofore, electrically operated pipettes having a stepper motor, such as disclosed in Nishi, U.S. Pat. No. 3,915,651, and Klein, U.S. Pat. No. 4,399,711, have required such significant amounts of power that power has been supplied by a circuit which is separate from the other components of the instrument. Combination of the circuit and the remainder of the components into a self-contained instrument would result in a bulky instrument which would not be portable in any practical sense. Nor have the power demands of known stepper motor circuits heretofore enabled an electrically operated pipette to be battery powered.
Also, Citrin, U.S. Pat. No. 4,369,665, discloses a detector for sensing an overcurrent condition of a motor to cause the motor to be de-energized immediately when a piston engages a discharge stop. This causes a cessation of further discharge motion before a repeat of an intake stroke is commenced with an initial drive of the piston against a gate element to establish the intake starting position. However, the detector can respond to resistance to piston movement caused by a clogged pipette tip, misalignment of the piston with the cylinder, or other impediments to the movement of the piston that occur during discharge of liquid, which can result in inaccurate initial positioning of the piston.
A further difficulty with the known pipette technology is that precise digital movement has not been applied to alleviate inaccuracies inherent in pipetting and/or titrating with an air displacement pipette having an electrically operated linear actuator, such as disclosed in Nishi, U.S. Pat. No. 3,915,651, Klein, U.S. Pat. No. 4,399,711, and Citrin, U.S. Pat. No. 4,369,665. For example, the configuration of the piston and cylinder mechanism provides accuracy only over a single limited range, which means that inaccuracy has resulted when the pipette is operated beyond the given range. Furthermore, inaccuracies resulting from surface tension, atmospheric pressure, and expansion and contraction of the air typically found in air displacement pipettes have heretofore not been addressed.